March 14. Or rather 3/14, the first few digits of pi, the ratio between the diameter and circumference of a circle. The number in decimal form never ends, but the first 100 digits look like this:
To celebrate, I could go see Life of Pi, but I already did. Nonetheless, that was a great movie, and it has at least one reference to the number.
But to really celebrate, having a pie chart related to pie, and itself a circle which, of course, incorporates pi, would be great.